Video: What Is Zika?

The Zika virus, linked to severe birth defects in thousands of babies in Brazil, is spreading "explosively" and could affect as many as 4 million people in the Americas, the World Health Organization (WHO) said Thursday. Director-General Margaret Chan told members of WHO's executive board that the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions and that she was convening experts to assess it.

"Last year, the virus was detected in the Americas, where it is now spreading explosively. As of today, cases have been reported in 23 countries and territories in the region," Chan said, promising that the WHO would act fast.

Last year the U.N. health agency was criticized for reacting too slowly to West Africa's Ebola epidemic, which killed more than 10,000 people, and promised to cut its response time.

"We are not going to wait for the science to tell us there is a link (with birth defects). We need to take actions now," Chan said.

There is no vaccine or treatment for Zika, which is like dengue and causes mild fever, rash and red eyes. An estimated 80 percent of people infected have no symptoms. The virus has been linked to babies born with abnormally small heads.

Developing a safe and effective vaccine could take a year, WHO Assistant Director Bruce Aylward said, and it would take 6-9 months just to confirm whether Zika is the cause of the birth defects, or if the two are just associated.

"In the area of vaccines, I do know that there has been some work done by some groups looking at the feasibility of a Zika virus vaccine. Now something like that, as people know, isgoing to be a 12 month plus time-frame," he said.

Chan said the WHO would convene an emergency committee meeting Monday to help determine its response level.

"The level of alarm is extremely high," Chan told the Geneva gathering.

As the virus spreads from Brazil, other countries in the Americas are likely to see cases of babies with Zika-linked birth defects, the head of WHO's Americas regional office told Reuters on Thursday.

Brazil has reported around 4,000 suspected cases of microcephaly, far more than in an average year and equivalent to 1-2 percent of all newborns in the state of Pernambuco, one of the worst-hit areas.

Brazilian President Dilma Rousseff said on Wednesday that the country must wage war against the mosquito that spreads the virus, focusing on eliminating the insect's breeding grounds.

The WHO's Chan said that while a direct causal relationship between Zika virus infection and birth malformations has not yet been established, it is "strongly suspected".

"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions," she said.

Health and law expert Lawrence Gostin of Georgetown University, who had urged the WHO to act, welcomed Chan's decision to convene an expert meeting, saying in a statement that it was "a critical first step in recognizing the seriousness of an emerging epidemic".

U.S. Senator Edward Markey called on the WHO and the U.S. Department of Health to explain how they were tackling the virus, since many U.S. travelers visit the region and more are expected to attend the 2016 Olympics in Rio de Janeiro.

"This is clearly an urgent issue for our government,” he said in his letter to the Department of Health.

The U.S. National Institutes of Health's Anthony Fauci, who helped brief U.S. President Barack Obama on Zika this week, said that federal health agencies were watching Zika but do not anticipate a major outbreak there.

"We will see little mini-outbreaks like in Florida or in Texas that can be well-controlled with mosquito vector control. Hopefully, we will not see anything worse than that," he told CBS News.

Asked about the risks for those traveling to Brazil for the summer Olympics, Fauci said that aggressively controlling mosquitoes there "is probably the best way".

The United States has two potential candidates for a vaccine for Zika and may begin clinical trials in people by the end of 2016, but there will not be a widely available vaccine for several years, U.S. officials said.

Reflecting traveler concerns over the virus, JetBlue Airways Corp joined a string of airlines on Thursday offering to refund customers with travel to areas impacted by the Zika virus, or to let them rebook flights.